Semiconductor device production lines presently utilize various types of commercially available photolithographic exposure apparatus to transfer images from a glass photo mask onto a photosensitive coated substrate via a projection optical system. At present, the typical photo masks used in such projection systems are usually in the form of a plate comprised of fused silica glass or quartz glass. These glass photo masks are positioned, in a projection system or apparatus, on a mask support or holding assembly usually comprised of a support frame, open in the center. The glass mask, is placed on the frame or support arms such that it is supported only by its edge regions so that the center portion of the mask is clear of any obstructions such that an optical beam can be passed there through so that the images on the mask can be reproduced in an photo-resist imaging layer deposited on the surface of a semiconductor wafer.
As semiconductor devices, became smaller, denser and more complex, the images on the masks became correspondingly smaller, denser, and more complex. To produce these smaller, denser devices the optics also improved to the point that the depth of focus required for the lithographic projection apparatus needed to produce such components has become so small that the flatness of the photo mask being used is no longer a negligible component. In an attempt to correct for this, mask blank suppliers have been making rapid advances in creating very flat photo mask blanks and have improved the flatness of the mask surfaces from 2.0 μm flatness to 0.5 μm flatness or better. However, the overall size of such masks is relatively large and, at present, such masks typically cover an area of 36 square inches or more. Such large photo masks thus must have a minimum thickness of between 6 or 7 mm to provide the mechanical strength necessary for their handling and to support themselves when placed on the spaced apart mask support arms. Thus even though the mask surfaces are now initially flatter and more uniform, the weight of large masks is such that when the mask is placed on separated mask supports, the unsupported center of the mask distorts, i.e. sags, due to gravity. The gravitational induced sag in the center of a 6.35 mm thick, six inch square mask supported at opposing edges is typically between 0.5 μm and 1.0 μm.
Any such sag in the center of the mask creates significant focusing errors in the projected image and some manufacturers, to correct for such focusing errors, have attempted to develop algorithms to optically correct for such focusing errors. One such technique attempts to do so by altering the focal point of the projection beam to compensate for the amount of sag in the surface of the mask However, to date, such wafer exposure programs and controls are not only very complex but in most cases only partially correct for the focus errors caused by gravitationally induced sagging.
Accordingly the present invention is designed to circumvent these difficulties and does so by providing a mask exposure apparatus that will mechanically apply forces to the mask that will cancel or substantially reduce the errors or distortions in the mask caused by gravitational forces.